[Sean O'Hagan is a journalist and writes for the Guardian.]

In 1969, a year after Russian tanks rolled into Prague, Josef Koudelka visited London with a Czech theatre group. One Sunday morning he was walking out of his hotel near the Aldwych Theatre when he saw some members of the theatre group perusing a copy of the Sunday Times magazine. As he passed, he saw to his surprise that they were looking at his own extraordinary photographs of that Russian invasion and the spontaneous street protests it provoked. The same photos have since become the definitive pictorial record of a pivotal event in 20th century history.

'They showed me the magazine where it said that these pictures had been taken by an unknown photographer from Prague and smuggled out of the country,' he says, shaking his head as if he still cannot believe it. 'I could not tell anyone that they were my photographs. It was a very strange feeling. From that moment, I was afraid to go back to Czechoslovakia because I knew that if they wanted to find out who the unknown photographer was, they could do it.'

Before he returned to Prague, Koudelka began making preparations to leave again, this time for good. He began by contacting the Magnum photographic agency, which had placed his work in the Sunday Times, attributing the images to PP (Prague Photographer) lest he and his family should suffer reprisals. Magnum subsequently wrote a letter to the Czech Ministry of Culture saying they had given him a grant to photograph Gypsies across Europe. It worked. In 1970, when his visa ran out, Koudelka did not return home. Suddenly stateless, he too became a kind of Gypsy, constantly on the move, forsaking the very notion of a homeland.

So began perhaps the most remarkable journey in 20th century photography. When I meet him today, in the back room of his new apartment in Prague, Koudelka unfolds a battered map of the world he has just found in one of the many boxes stacked along a wall. It is covered in spidery ink trails that trace his wanderings through Europe and beyond, his handwriting providing a runic commentary of the festivals and gatherings he attended along the way. The map dates from the Seventies and looks like a strange work of art, which, in a way, it is. The real art, though, lies in the photographs Koudelka produced when he began chronicling his restlessness - and rootlessness - as well as his newfound sense of freedom. His first major work, published in 1975, was called simply Gypsies, his second, from 1988, Exiles. Their titles alone tell you much about Koudelka's own life as well as the lives of his subjects.

'For 17 years I never paid any rent,' he says, laughing and raising a shot glass of slivovic, a plum brandy he has produced to welcome me to Prague. 'Even the Gypsies were sorry for me because they thought I was poorer than them. At night they were in their caravans and I was the guy who was sleeping outside beneath the sky.'

Now, 38 years after he began his exile, and 40 years after the invasion, Josef Koudelka has a place in Prague that, were he so inclined, he could call home. It is an apartment on the corner of a quiet square near the city centre, opposite a church where, he tells me proudly, the young Dvorak once wrote music. It is light, airy and purely functional: a place to work rather than rest. The boxes of notebooks, meticulously classified, suggest a highly ordered mind. 'Josef is utterly methodical, and utterly aware of his own legend and his legacy,' says his close friend, the photographer, Eliott Erwitt. 'But he is also an eccentric. He thinks differently, and, as we know from the work, he sees the world differently.'